1. Field of the Invention
This invention relates to repulpable wet strength paper and to a process for fmaking the paper.
2. Description of the Prior Art
Wet strength resins are often added to paper including paperboard at the time of manufacture. In the absence of wet strength resins, paper normally retains only 3% to 5% of its strength after being wetted with water. However, paper made with wet strength resin generally retains at least 10%-50% of its strength when wet. Wet strength is useful in a wide variety of paper applications, some examples of which are toweling, milk and juice cartons, paper bags, and liner board for corrugated containers.
As stated in Handbook for Pulp and Paper Technologists, Gary A. Smook, Angus Wilde Publications, 1992: xe2x80x9cPaper has traditionally been defined as a felted sheet formed on a fine screen from a water suspension of fibers. Current paper products generally conform to this definition except that most products also contain non-fibrous additives. Dry forming methods are now utilized for the manufacture of a few specialty paper products. Pulp is the fibrous raw material for papermaking. Pulp fibers are usually of vegetable origin, but animal, mineral, or synthetic fibers may be used for special applications. The distinction between paper and paperboard is based on product thickness. Nominally, all sheets above 0.3 mm thickness are classed as paperboard; but enough exceptions are applied to make the distinction somewhat hazy.xe2x80x9d
Because of increased commercial emphasis on developing paper products based on recovered cellulose, there is growing interest in developing paper which is readily repulpable. Paper and paperboard waste materials are difficult to repulp in aqueous systems without special chemical treatment when they contain wet strength resins.
Improving the repulpability of paper containing wet strength resins has generally been achieved by modifying the repulping conditions.
In U.S. Pat. No. 2,872,313, House et al. teach the use of hypochlorite salts to repulp paper containing polyaminoamide/epichlorohydrin wet strength resins. Miller, in U.S. Pat. No. 3,427,217, teaches the use of oxidizing salts such as sodium hypochlorite, ammonium persulfate and others, to repulp wet strength paper containing resins such as polyaminoamide/epichlohydrin resins, urea-formaldehyde and melamine-formaldehyde resins, etc. Although the hypochlorite salts effectively repulp wet strength paper, they are also chlorinating agents under some conditions and can form environmentally undesirable chlorine-containing degradation products in the process effluents.
Schmalz, in TAPPI, 44, no. 4, pp 275-280, April 1961, teaches the repulping of polyaminoamide/epichlorohydrin resin-containing paper with hypochlorite, or with strong alkali. Although the alkali method will not chlorinate organic byproducts, it is a relatively slow process.
Espy, in European Patent Application Publication No. 585,955-A, discloses a composition for repulping paper containing polyaminoamide/epichlorohydrin wet strength resins in an aqueous slurry, which comprises treatment with a non-chlorinated oxidizing agent and an alkali that is a water-soluble buffering salt capable of maintaining a pH of 7-12 in the aqueous reaction mixture.
Caropreso et al., in PCT International Publication No. WO 94/20682 disclose a composition containing a persulfate and a carbonate, bicarbonate or sesquicarbonate, which composition is suitable for oxidizing wet strength resin used in wet strength paper. The combination is stated to decrease the time required to repulp wet strength paper.
In U.S. Pat. No. 5,330,619, Henry et al. teach a method for treating fibrous sheet materials, such as paper or paperboard containing wet strength resin, which comprises reacting the fibrous materials with an enzyme to hydrolyze the resin and thereby improve repulping of the fibrous materials.
Copending application Ser. No. 08/657,242, filed Jun. 3, 1996 discloses a process for repulping paper by providing paper comprising at least one coacervate complex made from at least one cationic polymer and at least one anionic polymer and repulping the paper to obtain recycled pulp fibers.
Since many of the repulping processes used for wet strength paper result in formation of environmentally undesirable chlorine-containing degradation products, involve strong oxidizing agents, or proceed slowly, there is a need for improved methods for making wet strength paper that will be readily repulpable without significantly lowering the wet and dry strength properties of the paper.
According to the present invention there is provided a paper composition comprising: at least one cationic thermosetting resin comprising the reaction product of (i) at least one polyamide having secondary amine groups, made from reactants comprising polyalkylene polyamine, an acid component comprising at least 50 mole % of succinic acid or its ester or anhydride, and optionally at least one dicarboxylic acid (other than succinic acid) or its ester or anhydride, and (ii) epihalohydrin, said paper being repulpable substantially faster than paper that is substantially the same except that it contains conventional wet strength resin.
Further according to the present invention there is provided a process for making repulpable wet strength paper of comprising:
(a) providing an aqueous suspension of pulp fibers;
(b) adding to the aqueous suspension at least one cationic thermosetting resin comprising the reaction product of (i) polyamide having secondary amine groups made from reactants comprising polyalkylene polyamine, an acid component comprising at least 50 mole % succinic acid or its ester or anhydride and optionally at least one dicarboxylic acid (other than succinic acid) or its ester or anhydride, and (ii) epihalohydrin; and
(c) sheeting and drying the aqueous suspension to obtain wet strength paper that is repulpable substantially faster than paper that is substantially the same except that it contains conventional wet strength resin.
Cationic thermosetting resins suitable for use in the present invention comprise the reaction product of polyamides having secondary amine groups and epihalohydrin. The polyamides are made from polyalkylene polyamines, succinic acid or its ester or anhydride and optionally dicarboxylic acid (other than succinic acid) or its ester or anhydride.
The dicarboxylic acids contemplated for use in the present invention are saturated aliphatic dicarboxylic acids, preferably containing from 3 to 8 carbon atoms, such as: malonic, glutaric, adipic, and so on. Of these, the saturated aliphatic acids having from 4 to 6 carbon atoms in the molecule, such as glutaric and adipic are most preferred. Blends of two or more of these dicarboxylic acids may also be used, as well as blends of one or more of these with higher saturated aliphatic dicarboxylic acids, such as azelaic, sebacic, as long as the resulting long chain polyamide is water-soluble.
A variety of polyalkylene polyamines including polyethylene polyamines, polypropylene polyamines, polybutylene polyamines and so on may be employed herein of which the polyethylene polyamines represent an economically preferred class. More specifically, the polyalkylene polyamines of this invention are polyamines containing two primary amine groups and at least one secondary amine group in which the nitrogen atoms are linked together by groups of the formula xe2x80x94CnH2nxe2x80x94 where n is a small integer greater than unity and the number of such groups in the molecule ranges from two up to about eight and preferably up to about four. The nitrogen atoms may be attached to adjacent carbon atoms in the group xe2x80x94CnH2xe2x80x94 or to carbon atoms farther apart, but not to the same carbon atom. This invention contemplates not only the use of such polyamines as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, dipropylenetriamine, and the like, which can be obtained in reasonably pure form, but also mixtures and various crude polyamine materials. For example, the mixture of polyethylene polyamines obtained by the reaction of ammonia and ethylene dichloride, refined only to the extent of removal of chlorides, water, excess ammonia, and ethylenediamine, is a very satisfactory starting material. Most preferred are the polyethylene polyamines containing from two to four ethylene groups, two primary amine groups, and from one to three secondary amine groups, e.g. diethylenetriamine (DETA).
The term xe2x80x9cpolyalkylene polyaminexe2x80x9d employed in the claims, therefore, refers to and includes any of the polyalkylene polyamines referred to above or to a mixture of such polyalkylene polyamines and derivatives thereof.
It is desirable, in some cases, to increase the spacing of secondary amine groups on the polyamide molecule in order to change the reactivity of the polyamide-epichlorohydrin complex. This can be accomplished by substituting an aliphatic diamine such as ethylenediamine, propylenediamine, hexamethylenediamine or a heterocyclic diamine such as piperazine or the like for a portion of the polyalkylene polyamine. For this purpose up to about 60% of the polyalkylene polyamine may be replaced by a molecularly equivalent amount of the diamine. Usually a replacement of about 30% or less will serve the purpose.
The temperatures employed for carrying out the reaction between the dicarboxylic acid and the polyalkylene polyamine may vary from about 110xc2x0 C. to about 250xc2x0 C. or higher at atmospheric pressure. For most purposes however temperatures between about 160xc2x0 C. to about 210xc2x0 C. have been found satisfactory and are preferred. Where reduced pressures are employed somewhat lower temperatures may be utilized. The time of reaction depends on the temperatures and pressures utilized and will ordinarily vary from about xc2xd to 2 hours, although shorter or longer reaction times may be utilized depending on reaction conditions. In any event, the reaction is desirably continued to substantial completion for best results.
In carrying out the reaction, it is preferred to use an amount of dicarboxylic acid sufficient to react substantially completely with the primary amine groups of the polyalkylene polyamine but insufficient to react with the secondary amine groups to any substantial extent. This will usually require a mole ratio of polyalkylene polyamine (including aliphatic diamine if present) to dicarboxylic acid of at least about 0.9:1 more preferably at least about 0.92:1. However, mole ratio of at least about 0.8:1 may be used with quite satisfactory results. The mole ratio of polyalkylene polyamine to dicarboxylic acid can be up to about 1.4:1, preferably up to about 1.14:1 and more preferably up to about 1.2:1. Mole ratios outside of these ranges are generally unsatisfactory. Thus, mole ratios below about 0.8:1 result in a gelled product or one having a pronounced tendency to gel while mole ratios above 1.4:1 result in low molecular weight polyamides. Such products do not produce efficient wet-strength resins when reacted with epichlorohydrin.
Of the dicarboxcyclic acids employed in making the polyamide generally at least about 50 mole %, preferably at least about 80 mole % and most preferably about 100 mole % of succinic acid is used. The amount of the other dicarboxylic acid can be up to about 50 mole % and preferably up to about 20 mole %.
The reduced specific viscosity (RSV), which is an indication of molecular weight, for polyamide prepolymer derived from succinic acid and diethylenetriamine can be at least about 0.080, preferably 0.10 and most preferably 0.13 dL/g. The RSV for this prepolymer can be up to about 0.20, preferably 0.175 and most preferably 0.155 dL/g.
The preferred epihalohydrin for use in the present invention is epichlorohydrin.
In converting the polyamide, formed as above described to a cationic thermosetting resin, it is reacted with epichlorohydrin at a temperature from about 45xc2x0 C. to about 100xc2x0 C. and preferably between about 45xc2x0 and 70xc2x0 C. until the viscosity of a 20% solids solution at 25xc2x0 C. has reached about xe2x80x9cCxe2x80x9d or higher on the Gardner-Holdt scale. This reaction is preferably carried out in aqueous solution to moderate the reaction. pH adjustment is usually not necessary. However, since the pH decreases during the polymerization phase of the reaction it may be desirable, in some cases, to add alkali to combine with at least some of the acid formed.
When the desired viscosity is reached, sufficient water is then added to adjust the solids content of the resin solution to about 15% or less, the product cooled to about 25xc2x0 C. and then stabilized by adding sufficient acid to reduce the pH at least to about 6 and preferably to about 5. Any suitable acid such as hydrochloric, sulfuric, nitric, formic, phosphoric and acetic acid may be used to stabilize the product.
In the polyamide-epichlorohyrin reaction, it is preferred to use sufficient epichlorohydrin to convert all secondary amine groups to tertiary amine groups and/or quaternary ammonium groups including cyclic structures. However, more or less may be added to moderate or increase reaction rates. In general, one can use at least about 0.5 mole of epichlorohydrin per mole of polyamide secondary amine. It is preferred to use at least about 0.9 mole and most preferably at least about 1.0 mole of epichlorohydrin per mole of polyamide secondary amine group. Generally, up to about 1.8 moles, preferably up to about 1.5 moles and most preferably up to about 1.3 moles of epichlorohydrin per mole of polyamide secondary amine is used.
The process for manufacturing repulpable wet strength paper according to the present invention comprises a number of steps. One step comprises forming an aqueous suspension of pulp fiber which can be performed by conventional means, i.e., known mechanical, chemical and semi-chemical, etc., pulping processes. Another step comprises adding to the aqueous suspension of pulp fibers the cationic thermosetting resin described above. This can be done at any point, before sheet formation or it can also be applied after sheet formation from a tub size or at a size press or from showers to the dried or partially dried sheet. Yet another step comprises sheeting and drying the aqueous suspension of fine pulp fibers containing the cationic thermosetting resin. This can be done by conventional means.
The amount of cationic wet-strength resin can be as low as about 0.05 weight % on a dry basis, based on the weight of the dried paper, preferably about 0.1 weight % and most preferably about 0.2 weight %. The maximum amount of cationic thermosetting resin is generally about 2 weight % on a dry basis, based on the weight of the dried paper, preferably about 1 weight %, and most preferably about 0.5 weight %.
The process of repulping the paper to obtain recycled pulp fibers can be carried out by any mechanical action that disperses dry pulp fibers into an aqueous pulp fiber suspension. Conditions for repulping, as well as equipment commercially used, are discussed in xe2x80x9cHandbook for Pulp and Paper Technologists, Second Editionxe2x80x9d by G. A. Smook, Angus Wilde Publications, 1992, pp 194-195 and 211-212, which reference is incorporated herein by reference in its entirety.
A reproducible, quantative laboratory method to determine repulpability of wet strengthened paper uses a temperature controlled disintegrator described in TAPPI method T 205 OM-88, (1988), at 3000 rpm which is incorporated herein by reference in its entirety. Fiber yield (repulpability) is measured with a Somerville fractionating screen. Conditions during repulping are 1.5% consistency at 50xc2x0 C. and pH of 7 for 5 minutes. Using this test it was found that paper prepared by the process of this invention can be repulped in substantially less time than is required to repulp the same paper at about the same level of wet-strength, but containing conventional wet strength resin, e.g. Kymene(copyright) 557H aminopolyamide-epi wet strength resin.
Other wet-strength resin types that could be used in combination with the resins of the present invention as long as repulpability is not significantly adversely affected are: aminopolyamide-epi resins (e.g. Kymene(copyright) 557H-resin); polyamine-epi resins (e.g. Kymene(copyright) 736 resin), epoxide resins (e.g. Kymene(copyright) 450 and Kymene(copyright) 2064 resins); polyethylenimine, ureaformaldehyde resins; melamine-formaldehyde resins; glyoxalated polyacrylamides (e.g. Hercobond(copyright) 1000 resin, Parez 631NC); polyisocyanates; and reactive starches (oxidized starch, dialdehyde starch, blocked reactive group starch).
Other chemical additives that can be used in conjunction with wet-strength resins are: rosin size, reactive size (alkenyl succinic anhydride or alkyl ketene dimer), surface size, starch, retention aids, drainage aids, formation aids, flocculants, creping aids (adhesives and release agents), dry strength resins (cationic starch, guar gums, polyacrylamides), defoamers, scavengers for anionic trash and stickies control, fillers (clay, calcium carbonate, titanium dioxide), optical brightening aids and dyes.
The recycled pulp fibers prepared by the repulping process of the present invention can be used to make paper by conventional paper making processes, which comprise providing an aqueous suspension of the recycled pulp fibers and then sheeting and drying the aqueous suspension to obtain paper.
The wet strength resins of the present invention are suitable for use in the following areas: paper towels; napkins; facial tissue; liquid packaging board (milk carton, juice carton); poultry boxes; produce boxes; carrierboard; butchers wrap; bleached bag; poster board; table cloth; wallboard tape; currency paper; map paper; tea bag; corrugating medium; paper plates; molded products (egg cartons); laminating grades; flooring felt; coffee filter; bread wrap; multiwall bag; shingle wrap, etc.